1. Field of the Invention
The present invention relates to the field of precision instrumentation amplifiers that provide high gain accuracy and low input offset.
2. Prior Art
The accuracy of the DC output of a linear amplifier is subject to the physical imperfections of the individual devices comprising the amplifier. The errors caused by the imperfections are Input-Offset (VOS) and Gain Error (GE). The amplifier output (VO) is the algebraic sum of the amplifier Input (VI) and Input-Offset times the amplifier Gain (GA). The Input-Offset of the amplifier is the input required for a zero output. In practice the amplifier input offset is determined from the output of the amplifier for a zero input divided by the gain of the amplifier.VO=GA*(VOS+VI)VO=GA*VOS for VI=0or VOS=VO/GA for VI=0
Generally, an instrumentation amplifier has two differential input stages with low input bias currents. FIG. 1 shows one form of an instrumentation amplifier with differential inputs INP and INN, and output VO with output reference VREF, and with 2 gain setting resistors R1 and R2. The first differential input stage (GM1) is the amplifier input path (INP & INN), the second differential input stage (GM2) is functionally identical to GM1 and is the feedback path which takes a fraction of the amplifier output (VFB−VREF) and feeds it back. These two differential inputs are then subtracted to determine an error. The output stage (GM3) adjusts the output voltage VO to try to force the error to zero, which results in the amplifier differential input and the feedback differential input being equal. If GM1 and GM2 are identical and the feedback error is zero, the ratio of the output to the feedback input is equal to the ratio of the total resistance to the feedback resistance, which is the gain of the instrumentation amplifier.
In an integrated circuit, input offsets are a result of minor defects and minor device mismatches in amplifier gain stages. Methods for obtaining extremely low input offsets have been well understood and used for many years prior to the development of monolithic integrated circuits. These methods usually involve some form of or combination of chopper amplifier, chopper stabilization and auto-zero. These techniques are very amenable to semiconductor fabrication, are effective over a very wide temperature range, eliminate the need for offset trimming and lower the overall cost of production.
Conversely, methods to obtain gain accuracy have usually required the need for trimming, and usually do not obtain gain accuracy over a wide temperature range or a wide input common mode voltage range, where the input common mode voltage is the average of the two differential inputs relative to a common ground.
The lack of gain accuracy is referred to as Gain-Error (GE). One source of Gain Error involves the accuracy of the feedback. A resistor divider network in the output path usually determines the feedback differential input. The relative accuracy of this resistor divider ratio limits the gain accuracy of the amplifier. Semiconductor techniques of resistor matching and resistor trimming allow for very accurate control of resistor divider ratios over wide temperature ranges, which in turn are very effective in minimizing the adverse impact of resistor divider ratios on gain accuracy.
The other source of gain error is the relative lack of matching of the two differential input stages of the instrumentation amplifier. These two input stages take the differential voltage at their inputs and each creates an output that will be differenced with the output of the other input stage to generate an error signal. The outputs of these input stages may be a current or a voltage, but in either case if these input stages are not identically matched, there will be a resulting gain error. Here again, trimming has been used to improve the matching of the two differential input stages. There are severe limitations to the effectiveness of trimming because the source of mismatch is not from resistors but from active semiconductor devices operating over widely differing common mode input voltage conditions and over wide temperature ranges.
FIG. 2 is an example of a chopper-stabilized instrumentation amplifier. The upper circuit path is a continuous, high-speed instrumentation amplifier, and constitutes the main amplifier. Continuous here implies that the amplifier is a linear, continuous time architecture and that there is no interruption to the signal flow. The circles at the non-inverting inputs with the label VOS represent the fact that the amplifiers are not perfect and have a finite input offset. Only one input offset VOS is shown in each amplifier, even though the amplifier may have two parallel input stages, the VOS shown being the net difference between the two differential inputs. The lower circuit path is the chopper correction path. The blocks with the X represent the choppers that are simply series switches which during one half the chopper period are connected from the input straight across to the output, and during the other half of the chopper period are connected diagonally across. The amplifier between the chopper blocks is an instrumentation amplifier that outputs a differential current proportional to the difference between the two pairs of differential input voltages. The amplifier block with the capacitors connected from output to input is used as an integrator, and integrates the differential current from the chopped instrumentation amplifier output. The output of the integrator is a differential voltage and the amplifier stage to the right of the integrator is a simple V to I (i.e., voltage to current or transconductance) conversion stage which outputs a differential current proportional to the differential voltage at the integrator output. This differential current is the correction current which is applied to the main amplifier.
The main amplifier is high bandwidth and dominates the amplifier output at high frequencies while the correction current from the chopper path dominates at low frequencies due to the error integrator in the chopper path. Offsets are extremely low frequency effects and are thus minimized by the chopper path.
The chopper-stabilized amplifier is well established in practice, and in professional and patent literature. For more recent developments, see for instance, U.S. Pat. No. 7,132,883, entitled “Chopper Chopper-Stabilized Instrumentation and Operational Amplifiers”, and U.S. Pat. No. 7,209,000, entitled “Frequency Stabilization of Chopper-Stabilized Amplifiers”, both of which are assigned to Maxim Integrated Products, Inc., the assignee of the present invention, the disclosures of which are hereby incorporated by reference. In general, the chopper frequency selected will be dependent on many parameters in the design and application. A typical frequency might be 10 kHz to 50 kHz.
The imperfections of the instrumentation amplifier between choppers resulting in input offset errors are represented by the circles with VOS at the non-inverting inputs. The result of proper application of chopping is to reduce the effect of the input offsets by several orders of magnitude. Thus the chopper path is the low offset path and the output of the chopped instrumentation amplifier is integrated (i.e., error is continuously accumulated) providing correction to the main instrumentation amplifier. Once the two differential inputs of the chopped instrumentation amplifier are equal the differential output current is zero and the integrator maintains that value as the correction.